Brutman wrote:I should be able to do some testing for you. I don't think I've ever seen the 160x100 display mode - what is the advantage over the 160x200 display mode?
CommodoreJohn hit it on the head -- the 160x100 mode also works on a regular CGA card because it only needs 16K of video memory to work -- as opposed to 160x200 that takes 32k (which the CGA doesn't have).
For the most part it's just 80x25 text mode where you 'trick' the CRTC into thinking all characters are only two scanlines high -- so only the first two scanlines of each text character is shown. Nets you an 80x100 character display. You fill the screen with either one of the half-filled block characters (codes 0xDD or 0xDE) -- which puts half the character as filled and half showing the background -- background color becomes one pixel, foreground color becomes the other netting you 160 pixels across.
It's actually kind-of a sad state of affairs on the old PC's -- There is actually no reason a standard CGA couldn't have provided 320x200 16 color -- that's only 16k of RAM use and the 6845 can do that mode, but to save on card costs they didn't double-buffer the RAM meaning you had to interleave the pages for no snow in graphics mode, basically disabling half the on-board RAM in graphics mode. Really in terms of video hardware that's the ONLY difference between the Tandy/PcJr and a regular CGA card...
Though I remember back in the day a buddy of mine triple-piggy-backed another 16k on a CGA card to allow the same 160x200 mode as the Jr/Tandy.]
Brutman wrote:I'm assuming that it's not setting the 6845 registers in a nasty way. Also, have you tested this on a Jr yet? Some of the register locations are different, so if you have not consulted the port map this might be a dead end.
Only ports I'm playing with are 0x3D4/0x3D5 in terms of the video, which the Jr. does support... well, and port 0x60/0x61 for the keyboard and port 0x20 for acknowledging ISR's.
Vorticon wrote:It works great with no issues with the keyboard. My set up is a PCjr with V20 cpu, 640K RAM and the Tandy video modification. I wonder if any issues will arise without the latter, although the demo did state that CGA was detected.
Wait, did it say CGA or PcJr? If the former, then I have a bug somewhere.
-- edit -- found the bug, should be looking for $FD as the Jr's code, not $0F.
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